Fractionated local radiation therapy (RT) remains a primary treatment for many types of cancer. However for many large tumors, local failure is common. Strategies to successfully manipulate the tumor microenvironment to improve radiotherapy are under intensive investigation. Although ablative RT kill tumor cells directly we unexpectedly observed that initial tumor control following ablative RT is largely dependent on type I IFN and T cells but how irradiation induces IFN and how IFN helps T cells are less clear, RT-induced damage might also trigger tissue repair cascades, such as induction of TGFbeta, which induces M2-like macrophages which are associated with dampened RT initiated immunity potentially leading to tumor relapse. After initial screening with various cytokines and co-stimulatory molecules we find that local delivery of IL-21 after RT can improve microenvironment and controls tumor growth compared with either single treatment. We hypothesize that suppressive M2 macrophages post RT can be redirected to M1 macrophages to maximize anti-tumor immunity. In aim 1, we will define how ablative RT both positively and negatively regulates immune responses. On one hand, we will study how local RT breaks tumor barriers and induces DNA-cGAS-STING for type I IFN pathway. On the other hand, we will study how RT-mediated tissue damage gradually induces suppressive M2 macrophages for tumor relapse. In aim 2, we will study how IL-21 administration after RT can synergistically expand T cell responses and reduce RT resistance. We will study how RT-induced changes in microenvironment permits IL-21-Fc targeting to redirect suppressive M2 to supportive M1 macrophages that prevent relapse. We will determine whether such synergy depends on increased cytokines for macrophages and/or dendritic cells for cross-priming. In aim 3, we will generate a Her2/neu-dependent sporadic tumor. We then study whether and how RT can synergize with immunotherapy in genetically induced spontaneous tumor model or distal metastasis. We can thus further evaluate whether local RT can enhance Ab-IL21-mediated immunity against sporadic tumor model and RT-resistant or metastatic tumor. Understanding of changing tumor microenvironment after RT will help us design better combination strategies to control treatment-induced resistance and establish long- lasting protection.